Chapter 2 OLED - Organic Light Emitting Device
2.5 Full-color approaches in manufacturing
2.5.4 Down conversion of blue light
Emission of blue light from a layer can be filtered and produce red, green and blue. The method, shown in Fig. 2-8(b), converts blue to green and further green to red or directly converts blue to respective colors in terms of pre-patterned films of fluorescent material which efficiently absorbs blue light and re-emits the energy as either green or red light, depending on the compound used. Luminescent organic systems can have a conversion quantum efficiency approaching 100%, although the power efficiency is reduced since the energy of the emitted photon is of less than that of the absorbed photons. If necessary, color-filter may be adopted to sufficiently narrow the spectrum to achieve saturated color [26] [27].
2.6 Power efficiency in an OLED
The power efficiency is defined as the ratio between the power the display consumes and the amount of light emitted. For OLEDs, the power efficiency is of great concern because improved efficiency can lead to a longer device lifetime. The power efficiency for an OLED device is the same as the external quantum efficiency.
The external efficiency not only includes the internal efficiency but also takes outer factors as light emission from the side of the device and internal refraction into consideration. Internal quantum efficiency is defined as the amount of emitted photons in comparison to the amount of charges injected into the emitting layer. The internal quantum efficiency for OLEDs depends on the electron-hole recombination in the emitting layer, where an increased amount of recommendations leads to a better efficiency. It is therefore important to choose a material with good recombination property.
Even if all electrons and holes form an excited state, the quantum efficiency can not be theoretically higher than approximately 50%, depending on a fact that only the singlet excited states can contribute to the light emission. In conjugated polymers there are approximately 50% singlet and 50% triplet excited states [28]. Instead of emitting light, the triplet state emits heat that contributes to the degradation of the device. For small molecules there is about 25% singlet state so that the small molecule materials therefore inherently possess lower power efficiency compared to conjugated polymers [29][30][31]
.
According to the limitation of the quantum efficiency, due to the triplet excited state, an approach of improvement can be through doping with phosphorescent materials [28][ 32 ]. The contribution of a phosphorescent dye, such like platinum, accomplishes a mixture of the singlet- and triplet excited states, which generates a higher speed of the emission of light, known as phosphorescence [33]. Phosphorescent dopants enable small molecule OLEDs to have internal quantum efficiencies approaching 100%, as compared to an approximated 25% maximum for conventional fluorescent devices [34]. The increase in OLED efficiency directly translates into a reduction of display power consumption.
The conjugated polymers are considered to yield higher quantum efficiency due to the 1:1 singlet-triplet state ratio compared to small molecules with a singlet-triplet ratio of 1:3 [28][29]. It is considered that the conjugated polymers do not have any particular need for doping [35]. However, the internal quantum efficiency of a real device is much lower than the theoretical value but doping of the materials does enhance their performance. The reduction of the internal efficiency is mainly due to the absorption of the emitted light due to “Stokes shift” [36].
Table 2-1 shows a
comparison of luminous efficiencies for red, green and blue materials with their respective CIE coordinates, for the three main types of organic light emitting devices:phosphorescent device system [37], fluorescent small molecule materials [38] and spin coated polymer light emitting materials [39].
Table 2-1. Comparison of luminance efficiencies and CIE coordinates of phosphorescent OLEDs, fluorescent OLEDs, and polymer OLEDs (PLED)
[37][38][39].
Redcd/A, CIE
Green cd/A, CIE
Blue cd/A, CIE phosphorescent OLED 11, (0.65, 0.35) 24, (0.30, 0.63) 11, (0.16, 0.32)
fluorescent OLED 3, (0.63, 0.37) 7, (0.31, 0.63) 3, (0.15, 0.17) PLED 2, (0.60, 0.31) 13, (0.39, 0.59) 3, (0.15, 0.17)
2.7 The degradation process for OLEDs
The mechanisms affecting the degradation process are strongly linked to the physical properties of the materials used and result in different degradation properties between inorganic and organic LEDs [21]. The degradation mechanisms for OLEDs are not fully clarified in comparison to inorganic LEDs where the processes are more widely understood and the lifetime is much longer. Although the development of the OLED technology has resulted in a better lifetime, it is still much lower than that of
the inorganic LEDs. The difference is that the nature of the organic materials makes them more sensitive to environmental changes and degradation.
For OLEDs with lower power efficiency, a higher current density is needed in order to generate light. Only a few percentage of the power applied to the display results in an emission of photons, the rest is converted to heat. Heat generated in the device causes degradation of the display. The increase of heat has been found to be proportional to the driving current of the display, consequently the development of more power efficient displays should result in a reduction of the heat developed, thus, a longer lifetime.
The degradation process depends on which color the material emits. To emit a blue color, a material with a larger band gap is needed, which makes it harder to inject charged carriers into display and consequently more energy is required. The high amount of current density makes the display more fragile and therefore results in a faster degradation for blue than for red and green emitting materials. The problem of high luminance value and lifetime is more evident when constructing color displays than monochrome displays. The large reduction of the light for color displays is a result of the color-filter used and reduces up to 90% of the display light intensity [23].
At the interfaces of the different materials, diffusion of the materials can lead to a decline in effectiveness and lifetime. To prevent the diffusion, an intermediate protective layer can be used to separate two layers without affecting the charge transport between them. Another way is to choose a material with more stable properties in order to suppress the material diffusion. Some materials used for intermediate layers can even enhance the carrier transport by smoothing the barrier.
The most widely used layer materials for intermediate layer purpose is polymers, carbon or a thin layer of oxide [21].
An important factor that affects the lifetime is the encapsulation of the display.
An OLED exposed to air degrades in hours due to the oxidation. Encapsulation with an inert gas has to be used in order to protect the display from oxygen and humidity diffusing into the layers. The drawback of efficient encapsulation is the increased weight and the reduced flexibility of the display.
There are several indications for the degradation of a display [21]. As mentioned above, the emitted light from the display reduces over time and instead the applied current density on the device must increase to maintain the original emission intensity.
The decrease of brightness can occur in two different ways, either directly as the voltage is applied resulting in a short-term decrease or as slow decrease in brightness during the whole operating period of the display.
The degradation can also be noticed as the dark spots on the surface of the display. The phenomena of black spot can appear in both displays based on small molecules or conjugated polymers [40][41][42]
. The black spots are areas on the displays, which not emit any light and contributes to an overall decrease in light emission from the display. The spots are commonly situated at the interfaces between the different materials often between the metallic electrode and the organic layer. The position of the spots also reduces the interface effectiveness leading to the need of a larger input current. The occurrence of the spots is still not fully understood but some suggestions of their origin have been made. It is generally believed that some types of spots occur as a result of an electrical short, often with its origin at one of the two electrodes. The shorts appear as a result of defects and impurities in the different materials. These spots have a circular shape and can reach a size of 300 µm in diameter. Observations have been made in polymer based diodes where small black dots first appear on the periphery of a white dot in the centre. The black dots continue to grow until the whole
region within the periphery is covered with black spots, only leaving the initial white dot in the center.
In other cases, the spots appear a bubble-like structure as well. These spots appear suddenly and do not grow as in the case with the spots mentioned above. It was reported that these spots are a result of gases, mostly water, trapped in the spot [43]. The crystallization of some materials like the Alq3 is also assumed to be the source of black spots. Crystallization is overall believed to be a parameter that decreases lifetime due to changes in morphology and in some cases causing unwanted diffusion of the interfaces in the layered structure. The crystallization is believed to be a result of moderate heating during long operation periods or as a result of humidity in the device.
2.8 Factors that reduce and prevent the degradation in OLEDs
It is possible to reduce the degradation process as the organic materials with higher power efficiency have been developed. Various types of doping of the organic materials can result in higher power efficiency and longer lifetime [44]. As mentioned before, the triplet excited states are able to emit light instead of heat through phosphorescent doping, [37] which can not only yield a higher power efficiency but also reduce the time spent in the excited state. In the excited state, the molecules can be considered more reactive and can cause a degradation of the emitting layer due to unwanted chemical reactions. The reduction of triplet excited states would also lead to less heat generated and consequently to a better lifetime of the device.
Multi-layer structure can also bring a longer lifetime [24]. Depending on layers order designed, the lifetime can be possibly extended. The emitting layer plays a great role by affecting the lifetime as a result of its position. The addition of a hole
transporting layer enhances the lifetime by function as a stabilizer for the flow of holes and also in an overall better power efficiency due to more recombinations. Both transporting layers not only function as transport medium but also in some case function as buffer layers preventing humidity and oxygen to diffuse into the active emitting layer.
Due to the fact that OLEDs degrade under high temperatures, cooling is another effective method in order to achieve a longer lifetime. How the displays are fabricated is a factor in the desire of enhancing the lifetime, the process can be undertaken in vacuum or under great pressure or in some cases at low pressure, depending on materials used. During manufacturing the conjugated polymers and the small molecules are very sensitive to UV-light in combination with humidity. The fabrication has to be precise because the structure asymmetry and varying thickness of the layers are sources for local heating which can lead to damages on the display.
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Chapter 3
Addressing scheme for OLED displays
A display is an array of controllable pixels and the number of which depends on the dimension and resolution required by a particular application. For example, specifications of desktop monitor may emphasize higher visual performance, such as higher spatial resolutions and higher pixel content. The addressing of a large number of pixels in an array is an important issue in the display technology. Among the five addressing schemes used in electronic display [1], matrix addressing is the most suitable for OLED-based display system. In a matrix addressed display, pixels are arranged in rows and columns, and each pixel is electrically connected between one row electrode and one column electrode. The matrix addressing where active switch devices are added to the pixels are called active-matrix (AM) addressing. While the array without any active component in the pixels is termed as passive-matrix (PM) addressing.
3.1 Passive-matrix addressing
A passive-matrix array consists of two sets of electrically isolated conducting electrodes arranged orthogonally with an OLED to form the pixel at each intersection, and connected to the external drivers that supply the necessary voltage and timing sequence.
Fig. 3-1(a) and (b) show the electrical schematic and functional
cross-sectional diagram of a PM-OLED, respectively. Normally, the display is scanned or multiplexed row by row from the top to the bottom at a rate that is sufficient to produce flicker-free images (> 60Hz). To turn on a pixel, a certainvoltage needs to be decreased across the OLED material. The row electrode delivers a fraction of this voltage, and the column electrode provides the remainding.
(a) (b)
Fig. 3-1. (a) Schematic diagram of passive-matrix OLED panel. (b) Cross-section view of passive-matrix OLED structure.
A pixel receiving only part of the full voltage will be off. This row-at-a-time mode is chosen to maximize the pixel duty factor (defined as the percent of the total time each pixel is driven into the ON state by the column signal). The pixel duty factor of such a row-scanned array is 1/Ns, where Ns is the number of scan electrodes. Since the selected pixel must be driven with a pulsed voltage signal at a duty cycle, instantaneous luminance L0 should be high enough to achieve an average display luminance Ld:
d s L N
L0 = ⋅
Eq. 3-1
Even the EL response time of small-molecule OLED has been found to be < 1 µs [2], sufficient for most pulse-driven passive matrix, the number of rows in an array may limit the average display luminance [3][4]. However, this PM addressing approach limits the contrast and restricts the format of the display to smaller pixel counts [5]. For example, an instantaneous luminance should be about 10000 cd/m2 to achieve an
average luminance of 100 cd/m2 for a passive-matrix display with 100 rows, In addition, this approach requires patterning of both the row and column electrodes, which is difficult if the most common electron injecting materials (Al-Li, MgAg) is used as the column electrode. Besides, the high driving voltage and the instantaneous driving current corresponding to the high instantaneous-luminance requirement can also lower the OLED power conversion efficiency and OLED lifetime.
3.2 Voltage-type active-matrix addressing
Active-matrix addressing overcomes the crosstalk limitation of passive-matrix by integrating switching devices at the cross point of the row (scan or gate) and column (data) lines, and thereby isolating the off pixels from these select voltage lines. The TFT active-matrix array designs are commonly optimized using computer simulations to analyze electrical performance based on statistically extracted TFT and fabrication process parameters. While this approach is the most accurate way to predict the statistical mean and variance in display performance, it is more instructive to carry out a simple, physically based parameter analysis to identify functional dependencies, performance limits, and minimum requirements. The analysis presented here is applicable to any kind of TFT processing technology.
Using an active-matrix addressing can solve the image contrast and column electrode pattering concern of passive-matrix addressing. In the AM addressing, a transistor is placed at each pixel to separate the effect of the data line (column electrode) voltage and the scan line (row electrode) voltage on the voltage across the
Using an active-matrix addressing can solve the image contrast and column electrode pattering concern of passive-matrix addressing. In the AM addressing, a transistor is placed at each pixel to separate the effect of the data line (column electrode) voltage and the scan line (row electrode) voltage on the voltage across the